1. Field of the Invention
The present invention pertains to the synthesis of carbon nanotubes by catalytic chemical vapor deposition methods. Specifically, the present invention involves methods and apparatus, for making carbon nanotubes comprising the selective heating of catalysts and their products.
2. Description of Related Art
Carbon nanotubes (CNTs) are graphitic filaments/whiskers with diameters ranging from 0.4 to 500 nm and lengths in the range of several micrometers to millimeters. The designation “CNT” is often used to call all types of vertically aligned carbon tabular structures including single-wall CNTs, multi-wall CNTs, and carbon nanofibers. CNTs exhibit a variety of desirable and unique electronic and mechanical properties. For example, the thermal conductivity of CNTs is about twice that of diamond, and the electric-current-carrying capacity of CNTs is 103 higher than that of copper wires. In addition, the elastic modulus of CNTs is about double that of diamond and their strength is 10-100 times higher than that of the strongest steel. These useful properties of CNTs, coupled with their unusual molecular symmetry, have opened new frontiers in the manufacturing of electron field emission sources, nanodiodes, nanotransisters, biological probes, scanning probe microscopy tips, composite polymers, and hydrogen and energy storages (1).
Catalytic Chemical Vapor Deposition (CCVD) and Plasma Enhanced Catalytic Chemical Vapor Deposition (PECCVD) syntheses of CNTs are often used to synthesize CNTs directly on substrates. Both methods involve temperatures high enough (above 500° C.) to provide the energies needed for the chemical reactions that produce CNTs. In a PECCVD method, this heat is provided by plasma. Plasma-assisted growth of CNTs is typically conducted in a DC plasma reactor (2). Such a reactor comprises a grounded anode and a powered cathode. The wafer substrate used for the synthesis of CNTs is placed either on the anode or cathode. The electrode holding the wafer typically has a heating source which is used to increase the wafer temperature and consequently enable CNT formation. A disadvantage of DC systems is the formation of high negative bias on the wafer in these systems (>300 V). Recently, microwave reactors have come into use for the synthesis of CNTs. Plasma in such a reactor is sustained by the microwave source, and an additional DC or RF power supply may independently control the energy of ions striking the wafer.
High crystalline quality material can typically be produced only at very high temperatures, such as 500° C. or higher. This heating often damages the substrate or causes device integration problems (3-5). WO 03/011755 A1, incorporated herein by reference in its entirety, discloses a method for making CNTs on a substrate wherein the temperature of the substrate is maintained at a temperatures ranging from 30° C. to 300° C. This method uses a radio frequency (RF) source to generate plasma that enhances chemical vapor deposition by providing the energy required for the reactions that produce CNTs. One advantage disclosed for this method is the replacement of a filament used to generate plasma by a RF source. The amount of heat transferred to substrate by RF-generated plasma is less than that transferred from a filament, making it possible to maintain lower substrate temperatures during CNT synthesis.
The method of the present invention provides selective heating of only catalyst particles, thereby applying high temperature only to the catalyst surfaces where CNT-forming reactions occur and preventing excessive heating of the substrate. This is achieved by using one or both of two mechanisms of heating: (i) heating from exothermic reactions of catalytic oxidation on the surface of catalyst and (ii) heating from an RF source tuned to efficiently heat catalyst. This allows catalytically grown nanostructures on temperature-sensitive materials without damaging substrate structure. In contrast to WO 03/011755 A1, the present method does not require plasma or an RF source to generate plasma and uses a RF source tuned to a frequency that heats catalyst specifically and is dependent on the size of catalyst particles.